Carnitine is known to play an important role in human lipid metabolism. In cells, the carnitine is enzymatically bound to the fatty acids from fats and transports the fatty acids into the mitochondria, an organelle where fat is burned. Accordingly, the carnitine has a function as a carrier for the fatty acids and is essential to the lipid metabolism. The fatty acids are β-oxidized in the mitochondria and are converted into energy in the form of ATP in living body.
The lipids are important not only as an energy source but also as constituents of the human body. All cell membranes are composed of the lipids. On a macro scale, the lipids control evaporation of moisture from the body in the form of sebum and intercellular lipids.
Excessive lipids lead to obesity as a result of the accumulation of subcutaneous fat, and cause many cosmetic and QOL (quality of life) problems, including cellulite formation, shiny and greasy skin due to excessive sebum, seborrheic dermatitis and consequent hair loss, acnes, body odor, and skin aging due to decreased lipid metabolism.
As described above, the lipid metabolism requires that the fatty acids be transported into the mitochondria, for which the carnitine is responsible. Accordingly, the lipid metabolic rate is dependent on the amount of carnitine in the cells. Increasing the carnitine concentration in target tissues will stimulate the lipid metabolism and will prevent and eliminate excessive lipids and associated problems.
For such reasons, percutaneous absorption of various external skin preparations containing carnitine has been studied and proposed to stimulate the lipid metabolism (Patent Documents 1 to 4). However, such external skin preparations, which contain L-carnitine and salts thereof, have been unable to achieve satisfactory effects. The reason is probably that because the L-carnitine and salts thereof are hydrated quite easily, their direct use results in poor skin affinity and percutaneous absorption properties, and enough carnitine hardly reaches the tissue in which the lipid metabolism is to be performed.
Slimming external skin preparations are proposed, which contain a straight-chain acyl carnitine (in which the carnitine is modified with a straight-chain fatty acid residue) capable of higher dermatological effects than carnitine itself. As an example, a slimming external skin preparation contains palmitoyl-L-carnitine (L-carnitine palmitate), carnitine, caffeine and coenzyme A (Patent Document 5).
[Patent Document 1] Japanese Patent No. 3434995
[Patent Document 2] JP-A-H07-309711
[Patent Document 3] JP-A-2000-16916
[Patent Document 4] JP-A-2001-64147
[Patent Document 5] French Patent No. 2694195